The invention is based on a priority application EP08290631.4 which is hereby incorporated by reference.
The present invention relates to a method of and a base station for assigning precoding vectors in a mobile cellular network.
The technical field of the present invention is directed to cellular systems or wireless data systems, in which a base station (BS=base station) serves a number of mobile stations (MS=mobile station) on the same time/frequency resources by means of a MU-MIMO system (MU=multiple user, MIMO=multiple input multiple output). The MU-MIMO system is a system wherein a base station comprises N≧2 transmit antennas and serves a number of mobile stations on the same time/frequency resources. In order to allow each mobile station to receive only data streams intended for the mobile station, the base station has to apply certain antenna weights, also called precoding vectors, to the data streams that the base station transmits to the mobile stations.
In order to select the precoding vectors properly, the base station needs to have some channel state information. In TDD systems (TDD=time-division duplex), this channel state information can be obtained by uplink channel sounding because of the reciprocity of the uplink and downlink channels. In FDD systems (FDD=frequency-division duplex), however, this reciprocity is not available and can thus not be used. There is a need to define a fixed or preset set of preceding vectors. The set of precoding vectors is called code book. The code book is known to both the base station and mobile station. The mobile station selects a precoding vector that maximizes or minimizes some metric defining a signal quality parameter in order to improve its received signal quality. However, it is possible that a high intra-cell spatial interference still exists.